Discovering Earth’s History Rocks record geological events and changing life forms of the past. We have learned that Earth is much older than anyone had previously imagined and that its surface and interior have been changed by the same geological processes that continue today. Rocks Record Earth History
Discovering Earth’s History Uniformitarianism means that the forces and processes that we observe today have been at work for a very long time. A Brief History of Geology
How old is it??
There are two main ways to determine how old a rock or fossil is:
Relative dating tells us the sequence in which events occurred, not how long ago they occurred.
Chronological order - relational
Relative Dating—Key Principles
Principle 1: Law of Superposition
The law of superposition states that in an undeformed, undisturbed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it in the geologic column .
Index fossils are used to determine approximate age. Index fossils are widespread geographically, are limited to a short span of geologic time, and occur in large numbers.
What type of rock are they located in??
Ordering the Grand Canyon’s History
Principle 2: Original Horizontality The principle of original horizontality means that layers of sediment are generally deposited in a horizontal position.
Principle 2: Original Horizontality
Disturbed Rock Layers
What causes rocks to be disturbed? Unconformities • An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.
• An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred.
A Record of Uplift, Erosion, and Deposition
Absolute Dating with Radioactivity
Radioactivity - the spontaneous decay of certain unstable atomic nuclei
Radioactive Decay/Radiometric Dating
Dating with Radioactivity A half-life is the amount of time necessary for one-half of the nuclei in a sample (parent) to decay to a stable isotope (daughter). Half-Life
Dating with Radioactivity
Radiocarbon dating is the method for determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample.
When an organism dies, the amount of carbon-14 it contains gradually decreases as it decays. By comparing the ratio of carbon14 to carbon-12 in a sample, radiocarbon dates can be determined.